Project Summary/Abstract Fear is imperative to survival. As such, the brain has developed powerful and highly effective neural circuits that are capable of rapidly and flexibly organizing fear behaviors in response to threat. For this reason, when fears are formed inappropriately or expressed excessively, the consequences can be highly detrimental. This is exemplified by anxiety disorders such as post- traumatic stress disorder (PTSD), wherein following an extremely traumatic event, animals become highly sensitized and susceptible to the development and expression of a number of maladaptive behaviors. In contrast to adaptive associative fear memories or even specific phobias, the neural mechanisms underlying non-associative fear sensitization are poorly understood. In the research proposed here, we aim to investigate the neural circuits underlying trauma as well as trauma-induced enhancements in fear learning, elevated aggression, and disrupted sexual behavior. Aim 1 will combine cell-type specific functional manipulations with in depth behavioral analyses to examine the role of the dorsal bed nucleus of the stria terminalis (dBNST) in trauma. Genetically defined and anatomically restricted access to the dBNST will be obtained using Tac2-Cre mice and Cre-dependent viruses encoding optogenetic and chemogenetic effectors. Aim 2 will further dissect the unique microcircuits that control each trauma-induced behavioral phenotype by selectively manipulating and recording from downstream structures receiving Tac2+ dBNST input using optogenetics, CLARITY and fiber photometry. Building upon the tools and information obtained from Aims 1 and 2, Aim 3 will turn upstream, investigating the role of medial prefrontal cortex inputs onto dBNST Tac2+ cells and the ability for these inputs to exert top-down control of trauma and associated behaviors. Collectively, these aims will help to elucidate the neural circuitry that controls the fear-sensitized brain.